This invention relates to procedure and apparatus, e.g. furnace systems, for baking carbon bodies such as electrodes and components of electrodes, for use in molten electrolytes or for other electrical purposes. In an important specific sense, the invention is particularly concerned with the baking of carbon anodes, i.e. so-called pre-baked anodes, for conventional aluminum reduction cells wherein aluminum is electrolytically produced from alumina in a molten bath with accompanying reaction that progressively consumes the carbon. Carbon bodies of this sort are prepared of finely-divided carbonaceous solid such as petroleum coke, suitably calcined, or other carbon material equivalently purified, together with binder of the class of pitch and tar, the mixed mass being compressed to provide preliminarily coherent and self-sustaining bodies of rectangular, slab-like, block, brick or other desired shape, which require to be baked, e.g. at temperatures of about 1050.degree. to 1250.degree. C., to yield finished bodies of suitable hardness, firm coherence and impact strength for the intended handling and use, including resistance to thermal effects and erosion in molten alumina-salt baths.
So-called ring type baking furnaces are commonly used for the above operation, consisting of a honeycomb of rectangular refractory pits in which the carbons are baked, heat being applied to the carbons, for preheating and baking, and removed for cooling, by suitable gas flow through flues in the walls of the pits. In one conventional arrangement of such furnace, each open-topped pit can be about 0.5 to 1 m. in width, 3 to 5 m. long and 3 to 5 m. deep. Specifically, in small groups (e.g. 4 to 8) the pits are arranged side by side on a common level, with their long sides adjacent, each such group being called a section. The sections are arranged in a complete system, e.g. 16 to 80 in number, with the pits of each section being disposed endwise relative to the pits of the adjoining section or sections, so that the organization may provide from 4 to 8 rows with from 8 to 40 pits in each row in endwise succession, and with an equivalent series of pits arranged in parallel and connected by crossover flues so that the continuous ring may be completed. The flues being built in the longitudinal walls of each pit and being arranged for communication with the flues of the endwise adjoining pits unless deliberately blocked, the arrangement is such that each complete row may at a given time comprise one or preferably a plurality of long sets (say, 12 to 16 pits in each) of pits that succeed each other lengthwise, end to end.
Each such set, as defined at successive times in substantially continuous operation, can be considered a temporary baking unit, i.e. when the pits are loaded with carbon bodies. That is to say, three or four pits per row are subjected to preheating of green or unbaked bodies, two or three pits receive highest baking heat and six or seven are undergoing cooling, all by reason of the condition of the gas flowing in the sequence of flues along the pits, i.e. from the most cooled pit to the pit first subjected to preheating. Thus gas, preferably cold air, enters the flue system adjacent the last of the pits under cooling, passes the series of such pits and then the region of the final baking pits where high temperature heat, e.g. fire from burners, is injected into the gas stream; thereafter the very hot gas passes the preheating pits and is exhausted. For continuing operation, the circumstances of the pit-adjacent flue portions are altered intermittently: each 18 to 64 hours, the locality of fire injection is advanced a distance of one pit, concurrent with the direction of gas flow, and likewise the localities of air entrance and gas exhaust, whereby at each change a filled but unheated pit is added to and a pit with finished carbon bodies is removed from the sequence of pits or baking unit under treatment. In this way each filled pit is subjected to the entire series of steps, over a total period of many days.
In a practical furnace organization where the pits are arranged in sections of several pits each and many sections are disposed for lengthwise alignment of their pits, the complete structure provides in effect several rows of many endwise successive pits each, with heat-exchange gas flues, in effect, between the rows and along the outside rows. A plurality of temporary baking units can be arranged at any one time in each row, i.e. in succession, separated by several pits for which the flues are blocked and which are available for unloading baked and loading green carbon bodies. The flues have service ports adjacent the corresponding pits, which are blocked except when in use for entry of air or fire or for exhaust. Conveniently the fire burner means and the exhaust means, and if desired the air entry means, are arranged as manifolds crossing the array of rows and movable to successive positions along the array (including the return part of the ring), a number of such manifolds being provided whereby a number of successive temporary baking units can be set up in each row, and parallel such units in the several rows can be simultaneously advanced, section by section, as explained above.
Hence in each of the parallel working series or temporary baking units of pits as existing at any one time, the bodies in the pits next to the exhaust manifold are in the first stages of baking (preheating), those next to the fire manifold are in the final stages of baking and approaching their final baking temperature, while those near the end of the cooling section are cooled and ready to be unloaded. While the pits are of course stationary, each pit goes through all the stages in the cycle, which usually requires from about 10 to 30 days or more, a typical time being about 26 days for preheating, baking and cooling and for unloading and loading. Although in an overall sense the procedure is continuous, the baking capacity of a given furnace system is limited by the total time required to process each carbon body, and in the case of anodes for an aluminum smelter such capacity represents a very significant cost item in smelter operation.
Accordingly, it is important, if possible, to reduce the time for treating the carbon bodies and thus in effect produce more finished bodies per day. In the baking operation, the heating rate, especially for anode bodies, is important; thus in various furnaces heating rates average about 4.degree. to 12.degree. C./hour. Cooling the baked anodes (or other bodies) is generally not critical, and should be carried out as quickly as possible, to shorten the total cycle and achieve maximum production with a given furnace. Unfortunately, the cooling step is usually slow, chiefly because of the low heat carrying capacity of the cooling gas and low heat transfer coefficients associated with it.
A conventional mode of increasing the cooling rate is with forced air cooling, employing fans to pump large amounts of cold air into the last cooling sections. Since this is far too much to be utilized for combustion in the locality or localities of fuel injection for the baking fires, the hot air is largely returned into the building. This method has been found disadvantageous, notably in that large quantities of heat are simply exhausted into the atmosphere of the furnace building, and the so-extracted heat is lost. Dependent on the specific flue arrangement of the furnace, this type of extra cooling suffers in feasibility or efficiency; in a so-called horizontal flue furnace, the adverse effect of the forced air supply on the fire may require the fans to be placed too far from the fire to be of great use, while in vertical flue furnaces multiple fans can be used but if placed very close to the fire for maximum cooling, the correspondingly great amount of extracted heat is non-recoverable.